1. Field of the Invention
The present invention relates to display systems for use in cooperation with a computer or similar control system. The present invention further relates to means for adding a light pen capability to an existing display system under the control of a computer or similar control system.
2. Description of the Prior Art
Plasma display systems which rely on light emitted from an array of individual plasma discharge cells are now well known in the art. For example, U.S. Pat. No. 3,559,190 issued Jan. 26, 1971 to Bitzer et al. describes an early development in the field. In some respects plasma display panels are similar to well known cathode ray display systems such as those described in U.S. Pat. No. 3,653,001 issued Mar. 28, 1972 to W. H. Ninke and U.S. Pat. No. 3,389,404 issued June 18, 1968 to R. A. Koster. An important difference, however, between plasma display systems and CRT-based systems is that plasma displays have inherent memory, i.e., they need not be constantly refreshed by an information bearing sequence corresponding to the desired visual image. Thus, once a pattern of "on" and "off" cells is established, plasma display systems require only that there be applied to each on cell on a periodic basis a sustain signal to renew the discharge at operating cells or crosspoints. This sustain signal is not itself sufficient to cause breakdown. However, when breakdown has previously existed such pulses will cause the discharge to be maintained.
A useful adjunct of any computer-based display system is a so-called light pen for communicating to a computer or other control mechanism a location on the display surface. In typical CRT display systems, such as systems described in the Ninke and Koster patents, supra, a light pen is sensitive to the application of signals by the computer or similar device to the CRT. The computer then correlates the detection of the resulting light pulse and information stored internally relating to the refresh data causing the light to be emitted.
Because reference to the picture information is not always immediately available to the control computer for purposes of correlation as in CRT systems (because the information need not be available for refresh purposes), plasma display systems have used a slightly different arrangement. In general, a separate scanned pulse is used to generate a corresponding identifiable light pulse which can be detected by the light pen. For example, in accordance with my earlier invention described in U.S. Pat. No. 3,651,509 issued Mar. 21, 1972 I provide a system for effectively combining a light pen with a plasma display system. It will be noted, however, that the system described in this earlier patent requires the addition of a moderate amount of special purpose circuitry. Further, for some applications operating margins are found to be less than optimum.
My copending application Ser. No. 345,893 filed Mar. 29, 1973 now U.S. Pat. No. 3,851,327 issued Nov. 26, 1974 describes a light pen detection system using a specially timed scanning erase pulse to "flash" each on cell in a plasma panel. Because of the special timing of the resulting light pulse, it is possible to uniquely identify a given on cell using a standard light pen and simple logic circuitry. Because of the inherent memory in a plasma display cell, a scanned erase pulse does not sufficiently discharge the cell before the normal sustain signals arrive to again restore the normal charge distribution. It should be clear, however, that the system described in the last-mentioned patent application is not appropriate for detecting the location of a light pen in a position adjacent only off cells.
It is therefore an object of the present invention to provide means for identifying a particular on or off cell in a plasma panel array using a standard light pen.
An important limitation in using standard plasma panels for light pen detection and similar techniques is the typically narrow operating margins for address and sustain signals. Thus, for example, in connection with the light pen detection system described in my previous application Ser. No. 345,893 filed Mar. 29, 1973, and in my earlier invention described in U.S. Pat. No. 3,651,509, some care must be exercised in the choice of addressing pulse amplitude, duration and shape to insure that spurious writing and erasing of unselected cells does not occur.
It is therefore a further object of the present invention to improve operating margins for scanning pulses in a light pen detection system.